Highly-sensitive displacement-measuring optical device

ABSTRACT

Micron-scale displacement measurement devices having enhanced performance characteristics are disclosed. One embodiment of a micron-scale displacement measurement device includes a phase-sensitive reflective diffraction grating for reflecting a first portion of an incident light and transmitting a second portion of the incident light such that the second portion of the incident light is diffracted. The device further includes a mechanical structure having a first region and a second region, the mechanical structure positioned a distance d above the diffraction grating, the second portion of the incident light is reflected off of the first region of the structure such that an interference pattern is formed by the reflected first portion and the reflected second portion of the incident light. The device can further include an electrode extending toward, but spaced a distance away from, the second region of the mechanical structure.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation-in-Part application of U.S. utilitypatent application “Highly-Sensitive Displacement-Measuring OpticalDevice,” having Ser. No. 10/704,932, filed Nov. 10, 2003 now U.S. Pat.No. 7,116,430, which claims priority to U.S. provisional application“Miniature Diffraction-Based Optical Sensors,” having Ser. No.60/424,810 filed Nov. 8, 2002, and to which claims priority to, and is aContinuation-in-Part of, “Microinterferometers With PerformanceOptimization,” having Ser. No. 10/112,490, filed Mar. 29, 2002 (now U.S.Pat. No. 6,753,969), each of which are incorporated by reference intheir entirety.

This application is also related to U.S. utility patent applicationentitled “System and Method for Surface Profiling,” having Ser. No.10/113,362, filed Mar. 29, 2002.

This application is related to co-pending commonly assignedNon-Provisional Application entitled, “System and Method for SurfaceProfiling,” filed concurrently herewith on Apr. 17, 2006, and accordedSer. No. 11/405,053.

TECHNICAL FIELD

The present disclosure generally relates to measurement devices. Morespecifically, the disclosure relates to highly-sensitive, micron scale,displacement measurement devices.

DESCRIPTION OF THE RELATED ART

Hearing aids, for example, provide specific applications in whichmicron-scale displacement measurement devices may be used. Tinymicrophone arrays are currently housed in hearing aids to pick up slightacoustic pressures. Today's microphones measure a change in capacitancebetween two conducting plates, one of which (the microphone diaphragm)moves as a function of the acoustic pressure applied.

There are various drawbacks to today's micromachined capacitivemicrophones. The electrical sensitivity of the microphone, S_(e), isdefined as the change in voltage output per change in membranedisplacement. In a similar fashion, the mechanical sensitivity, S_(m),is defined as the change in membrane displacement per change in appliedpressure (i.e. S_(m) is simply the compliance or softness of themembrane). The total sensitivity of the device to sound pressure canthen be expressed as S_(e)×S_(m), with units of Volts/Pa. For highS_(e), a large DC bias voltage should be applied and the gap heightbetween the electrodes should be made as small as possible, typically onthe order of 2 μm. These attributes are, in fact, in conflict, as themaximum DC bias that can be used is limited by the electrostaticcollapse voltage, which decreases with shrinking gap height. Theelectrical sensitivity is a maximum when the device is biased near thiselectrostatic collapse voltage. Unfortunately, the detection schemebecomes nonlinear under this same condition. In addition to thisdrawback, implementation of the constant charge condition can requirethe use of high impedance amplifiers, which come with high electronicnoise. To compensate for poor S_(e), micromachined capacitivemicrophones use large, soft membranes on the order of 1-5 mm to enhancemechanical sensitivity and, in turn, the overall device performance.Even this approach is limited, however, by membrane stresses that resultduring fabrication. These stresses bound the mechanical sensitivity thatcan be achieved and make the fabrication of uniform membranes with highyield difficult. In addition to using a soft membrane, the backelectrode can be perforated and open to a large backside cavity toprevent additional stiffening which would otherwise occur fromcompression of the air in the thin gap. The perforation reduces theactive capacitance and adversely affects S_(e), leading to yet anotherdesign conflict. In summary, the electrical and mechanical sensitivityin a micromachined capacitive microphone are not independent and imposesevere design and fabrication limitations.

Optical interferometry is the act of splitting and recombiningelectromagnetic waves, in particular, visible light waves, to measuresurface geometries, distance, etc. The advancement in interferometry hascome in many avenues of technology. Long-range telescopes,high-precision spectrometers, compact disc players, etc., use some formof interferometry. Micro-machinery is a growing technology field thatoften utilizes interferometers because they typically have highresolution and precision. In general, displacement measurements in thesub-nanometer range can be detected with today's interferometers. Toexamine microscale structures, the lateral resolution of theinterferometers, generally, need to be improved. This can be achieved bycoupling the interferometer to a regular microscope. Unfortunately, thesize of the interferometer becomes rather large and subsequently may notfit in small spaces for inspection. Furthermore, to inspect a largenumber of microscale structures either the sample or microscopeobjective is scanned, resulting in slow imaging.

In order to obtain interferometric measurement sensitivity in a smallvolume, several methods have been developed. One of these methodsinvolves phase sensitive diffraction gratings as described in atechnical paper entitled “Interdigital cantilevers for atomic forcemicroscopy,” published in Appl. Phys. Lett., 69, pp. 3944-6, Dec. 16,1996 by S. R. Manalis, S. C. Minne, A. Atalar, and C. F Quate and alsoin U.S. Pat. No. 5,908,981 to Atalar et al.

Similar structures are also used in microaccelerometers to measure thedisplacement of a control mass with interferometric precision asdescribed in a paper written by E. B. Cooper, E. R. Post, and S.Griffith and entitled “High-resolution micromachined interferometricaccelerometer,” Appl. Phys. Lett., 76 (22), pp. 3316-3318, May 29, 2000.Both of these papers are incorporated by reference in their entireties.

Based on the foregoing, it would be desirable to incorporate opticalinterferometry with micro-machined microphone technology so as toimprove over the current prior art. Furthermore, it would be desirableto explore other aspects in which highly-sensitive, micron-scaledisplacement measurement devices may be utilized.

SUMMARY

Micron-scale displacement measurement devices having enhancedperformance characteristics are disclosed.

An embodiment of such a device includes a phase-sensitive reflectivediffraction grating for reflecting a first portion of an incident lightand transmitting a second portion of the incident light such that thesecond portion of the incident light is diffracted. The device furtherincludes a mechanical structure having a first region and a secondregion, the mechanical structure positioned a distance d above thediffraction grating, the second portion of the incident light isreflected off of the first region of the structure such that aninterference pattern is formed by the reflected first portion and thereflected second portion of the incident light. The device furtherincludes an electrode extending toward, but spaced a distance away from,the second region of the mechanical structure.

An embodiment of a method includes illuminating a reflective diffractiongrating with an incident light, the diffraction grating being positioneda distance d from a mechanical structure having a first region and asecond region, a first portion of the incident light being reflected anda second portion of the incident light transmitted through thediffraction grating such that the second portion of the incident lightis diffracted. The method further includes reflecting the second portionof the incident light off of the first region of the mechanicalstructure such that an interference pattern is formed by the reflectedfirst portion and the reflected second portion of the incident light.The method further includes applying a voltage bias to an electrodeextending toward, but spaced a distance away from, the second region ofthe mechanical structure.

An embodiment of device includes means for illuminating a reflectivediffraction grating with an incident light, the diffraction gratingbeing positioned a distance d from a mechanical structure having a firstregion and a second region, a first portion of the incident light beingreflected and a second portion of the incident light transmitted throughthe diffraction grating such that the second portion of the incidentlight is diffracted. The device can further include means for reflectingthe second portion of the incident light off of the first region of themechanical structure such that an interference pattern is formed by thereflected first portion and the reflected second portion of the incidentlight. The device can also include means for applying a voltage bias toan electrode extending toward, but spaced a distance away from, thesecond region of the mechanical structure.

Other devices, methods, features, and advantages of the presentinvention will be or become apparent to one with skill in the art uponexamination of the following drawings and detailed description. It isintended that all such additional systems, methods, features, andadvantages be included within this description, be within the scope ofthe present invention, and be protected by the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the disclosure can be better understood with referenceto the following drawings. The components in the drawings are notnecessarily to scale, emphasis instead being placed upon clearlyillustrating the principles of the present invention. Moreover, in thedrawings, like reference numerals designate corresponding partsthroughout the several views.

FIG. 1 is a diagram illustrating the concept of using a diffractiongrating to split beams in a microinterferometer.

FIG. 2 is a graph illustrating the distribution of reflected lightmeasured on an observation plane with various gap thicknesses utilizingthe method illustrated in FIG. 1.

FIG. 3 is a graph illustrating the normalized intensity of variousdiffraction orders vs. gap thickness utilizing the method illustrated inFIG. 1.

FIG. 4 is a diagram illustrating an embodiment of a micro-displacementmeasurement device in accordance with the present disclosure.

FIG. 5 is a diagram illustrating another embodiment of amicro-displacement measurement device in accordance with the presentdisclosure

FIG. 6 is a diagram illustrating another embodiment of amicro-displacement measurement device in accordance with the presentdisclosure.

FIG. 7 is a diagram illustrating yet another embodiment of amicro-displacement measurement device in accordance with the presentdisclosure.

FIG. 8 is a flow chart illustrating a method for optimizing thesensitivity of a micro-displacement measurement device and moregenerally improving the overall performance of a micro-displacementmeasurement device in accordance with embodiments of the presentdisclosure.

FIG. 9 is a diagram illustrating an embodiment of a micro-displacementmeasurement device in accordance with the present disclosure and havingan increased air volume for improving the sensitivity of the device.

FIG. 10 is a diagram depicting a side, cut-away view of an embodiment ofa micro-displacement measurement device in accordance with the presentdisclosure and having an open-gap structure for improving thesensitivity of the device.

FIG. 11 depicts a top view of an embodiment of a rectangular-shapeddevice having an open-gap structure of FIG. 10.

FIG. 12 depicts a top view of an embodiment of a circular-shaped devicehaving the open-gap structure of FIG. 10.

DETAILED DESCRIPTION

As will be described in greater detail herein, displacement measurementdevices in accordance with the present disclosure can measure the changein position of a membrane as a function of time due to a variety offactors. Furthermore, the displacement measurement devices can beoptimized for displacement sensitivity and reduced noise causing agreater signal to noise ratio (SNR).

Referring now in more detail to the drawings, FIG. 1 is a diagramillustrating the concept of using a diffraction grating to split beamsin a microinterferometer. This concept has been utilized in measuringprecise relative displacements, such as for the measurement of AtomicForce Microscopy (AFM) tip displacement and in spatial light modulators,as in the grating light valves (GLV). This concept is also disclosed inU.S. Pat. No. 6,567,572 entitled “Optical Displacement Sensor” to F. L.Degertekin, G. G. Yaralioglu, and B. Khuri-Yakub, which is incorporatedby reference in its entirety. AFM, in general, is a technique foranalyzing the surface of a rigid material at the atomic level. AFM usesa mechanical probe to magnify surface features up to 100,000,000 times,and it can produce 3-D images of the surface. In general, a GLV containsseveral tiny reflective ribbons that are mounted over a silicon chipwith a tiny air gap in between the chip and the ribbons. When a voltageis applied to the chip below a particular ribbon, that ribbon bendstoward the chip by a fraction of a wavelength of an illuminating light.The deformed ribbons collectively form a diffraction grating and thevarious orders of the light can be combined to form the pixel of animage. The shape of the ribbons, and therefore the image information,can be changed in as little as 20 billionths of a second.

The diagram of FIG. 1 illustrates two scenarios. A first scenario 1shows what occurs when a target surface 4 is placed a distance of ahalf-wavelength, λ/2, away from a reference point, in this case, areflective diffraction grating 5. A second scenario 2 shows what occurswhen the target surface 4 is placed a distance of a quarter-wavelength,λ/4, away from the diffraction grating 5. The detailed diffractionpattern of such a structure can be found by applying standarddiffraction theory to determine the locations and the dimensions of thephoto-detectors or light guide apertures.

In both instances, the reflective diffraction grating 5 is formed on atransparent substrate 3. Exemplary materials that may be utilized toconstruct such elements will be discussed in further detail in relationto FIG. 4. The diffraction grating 5 is formed of an array ofdiffraction grating fingers 6 equally spaced along a front edge of thetransparent substrate 3. It should be noted that, as mentioned above,this diagram is not to scale, and is merely for illustrative purposes.In reality, the diffraction grating fingers 6 would typically have aheight on the order of micro- or nano-meters.

In the first scenario 1, when an incident light is illuminated throughthe transparent substrate 3, a first portion of the incident light isreflected from the reflective diffraction grating 5. A second portion ofthe incident light is transmitted and diffracted about the diffractiongrating fingers 6. The transmitted and diffracted light reflects off ofthe target surface 4 and is measured by a proper detection unit (notshown), such as a photo-detector or a photo-diode. As in scenario 1, thetarget surface is placed at a distance of λ/2 or any integer multiple,thereof. In this case, the 0^(th) order of the transmitted incidentlight is reflected back. In general, the 0^(th) order is the transmittedlight that is illuminated directly, in which case no diffraction, orchange in direction occurs. The first portion of the incident light, andthe second portion of the incident light which has been reflected off ofthe target surface 4 interferes with each other. The phase of the twoportions of the light waves help form constructive and destructiveinterference patterns. From the interference patterns, the relativedistance between the diffraction grating 5 and the target surface 4 canbe determined.

In scenario 2, the same general structure is set up. In this case, thetarget surface 4 is placed a distance of λ/4 away from the diffractiongrating 5. In practice, the target surface 4 may be placed at anyinteger multiple of λ/4 and the same general results will occur. Whenthe first portion of the incident light joins with the second portion ofthe incident light upon reflection, destructive interference cancels outthe two. The second portion of the light travels an extra distance of 2×the distance between the target surface 4 and the diffraction grating 3,which results in a phase difference between the two portions of π,complete destructive interference. On the contrary though, the higherorder diffraction fields, such as the first order, can constructivelyinterfere with the first portion of the incident light. As FIG. 1depicts, the higher order first and second portions of the incidentlight are angled and not parallel to the line of illumination, like the0^(th) order beam.

Having described an example of using a diffraction grating to splitlight beams and therefore measure relative distance, theoreticalcalculations will be utilized to show the results of using the methoddescribed in FIG. 1. Suppose an incident light of λ=632 nm isilluminated through the transparent substrate 3 onto the reflectivediffraction grating 5. A laser can be utilized to provide the incidentlight. In this case, a helium-neon (HeNe) laser can be utilized. Supposethe diffraction grating 5 contains 10 diffraction grating fingers 6equally spaced at d_(g)=2 μm. FIG. 2 is a graph 10 illustrating thedistribution of reflected light measured on an observation plane withvarious gap thicknesses utilizing the method illustrated in FIG. 1.Hereinafter, the distance between a reference point, in this case thediffraction grating 5, and the target surface 4 will be referred to asthe gap thickness and can be considered the absolute distance to thesurface.

FIG. 2 shows the normalized intensity of reflected light 20 versus anobservation length, x 18. The observation length, x, is in the lateraldirection, and centered at the 0^(th) order beam. In this case, a 100 μmwide photo-detector has been used. Three scenarios are shown in thegraph 10. Scenario 12 shows the normalized intensity 20 with gapthickness, d=λ/2. Scenario 14 shows the normalized intensity 20 with gapthickness, d=λ/4. Scenario 16 shows the normalized intensity 20 with gapthickness, d=λ/8.

As expected, scenario 12 shows the 0^(th) order reflected beam withcomplete constructive interference. The higher order beams, e.g. the1^(st) and 3^(rd) order beams, incur destructive interference and sotheir intensity is cancelled out. Scenario 14 shows that the 0^(th)order has been completely cancelled out and the 1^(st) and the 3^(rd)orders of the reflected beam appear to have partial intensity. Scenario16 shows that when the gap thickness, d=λ/8, both the 0th order and the1^(st) order contain some light intensity. Perhaps, most importantly,graph 10 attempts to show the periodic nature of the intensity of theorders of the reflected light versus varying gap thickness.

The intensity of these orders as a function of grating-reflectingsurface shows the cos²(2πd/λ) and sin²(2πd/λ) variation, as illustratedin FIG. 3. FIG. 3 is a graph 30 illustrating the normalized intensity 38of various diffraction orders 32 and 34 versus gap thickness 36utilizing the method described in FIG. 1.

As shown in FIG. 3, the 0^(th) order curve 32 takes on a cos²(2πd/λ)shape. This is in line with the results found in FIG. 2. At gapthickness of λ/2, which is approximately 0.316 μm, the intensity isgreatest. At gap thickness of λ/4, which is approximately 0.158 μm, theintensity is zero. The 1^(st) order curve 34 takes on a sin²(2πd/λ)shape. The graph 30 of FIG. 3 clearly displays the periodic nature ofthe diffraction orders. As one can see, keeping all other variablesconstant and known, one can calculate the relative distance by measuringthe intensity of the orders, in particular the 1^(st) order. In fact, bymonitoring the intensity of any of the reflected orders, one can achieveinterferometric resolution on the order of 1×10⁻⁵ Å/√Hz.

The present disclosure provides a sensitive diffraction based opticaldisplacement apparatus and method to measure the static and dynamicdisplacement of reflectors in various applications. The presentdisclosure includes the use of any type of reflector, however, in apreferred embodiment, the present disclosure includes a flexible andoptically reflective membrane. The apparatus and method of the presentdisclosure is amenable to integration of electronics and optics to formcompact displacement detectors for a single membrane or membranesfabricated in the form of arrays. Typical applications of the presentdisclosure would be in, for instance, but not limited to, microphones(micro-machined or not), micro-machined ultrasonic transducers,micro-machined ultrasonic wave generators, micro-machined ultrasonicimage applications, pressure sensors and hearing aids. The embodimentsof the present disclosure would also be useful in any sensingapplication where the position of a reflector or membrane is changed dueto a chemical or physical process and this displacement needs to bemeasured accurately in a broad frequency range. Furthermore, theembodiments of the present disclosure would also be useful in anysensing application where the reflectivity of a reflector or a membraneis changed due to a chemical or physical process and this change needsto be measured accurately in a broad frequency range.

U.S. Pat. No. 6,567,572 entitled “Optical Displacement Sensor” disclosesvarious embodiments of a micro-machined optical displacement sensor. Forexample, FIG. 4 of U.S. Pat. No. 6,567,572 illustrates an opticaldisplacement sensor that includes a reflective diffraction grating 430deposited upon a transparent substrate 420. Suspended above thediffraction grating 430 is a reflector 410. A voltage bias is appliedacross the reflector 410 and diffraction grating 430 so as toelectrostatically actuate the reflector 410 to vary its position and/orrigidity.

An improvement to the sensor of U.S. Pat. No. 6,567,572 is embodied inthe present disclosure. That is to add a semi-transparent layer to thetop of the transparent substrate 420. This mirror layer can be builtusing a thin metal film and a dielectric stack of alternatingquarter-wave thick media. Preliminary testing shows the thickness of themirror layer and the reflectivity of the metal used can greatly increasethe sensitivity of the sensor. To complement the mirror layer placed onthe transparent substrate 420 would be the reflective surface of thereflective membrane 410. Similar testing of the thickness of themirrored reflective layer of the membrane 410 and the material usedproved complimentary results, however, keeping in mind that addedthickness and the various materials used can affect the rigidity of thereflective membrane 410.

FIG. 4 is a diagram illustrating a first embodiment of amicro-displacement measurement device 100 in accordance with embodimentsof the present disclosure. As shown in FIG. 4, the embodiments of thepresent disclosure provides for the use of a tunable, phase-sensitivediffraction grating 156 mounted above substrate 160 to measure thedisplacement of the membrane structure 150. A light source 140 isintegrated in the device 100 to provide an incident light beam to beshined on the reflective diffraction grating 156 and the reflectivemembrane 150. A photo-detector 120 is positioned to receive theinterference pattern produced by the reflected light beams. Theembodiments of the present disclosure are not limited to the number ofphoto-detectors. In addition, the present disclosure includes anotherembodiment where the photo-detectors are one or more optical fibers. Themain part of the device 100 consists of a flexible optically reflectivemembrane 150 suspended above a non-moving, transparent substrate 160.For displacement detection, the intensity of the light 101 from lightsource 140 reflected from the membrane 150 and reflective diffractiongrating 156 is monitored. Grating 156 consists of periodic reflectivefingers positioned atop transparent, yet conductive, electrodes 180which are deposited atop the transparent substrate 160. Displacement ofmembrane 150 changes the intensity of the diffraction orders 104 whichcan be easily detected using standard photo-detector(s) 120. Thisprovides the sensitivity of an optical interferometer, and has a betternoise performance as compared to other intensity based commercialoptical microphones.

Light source 140 provides incident light beam 101 through substrate 160to reflective membrane 150. Reflective membrane 150 is positioned todiffraction grating 156 at an odd multiple of a quarter of thewavelength of light source 140. In addition, the device 100 couldinclude a wafer (not shown), preferably a silicon wafer for housing thelight source 140 and/or the photo-detector 120.

In order to realize a sensitive microphone one should have a compliantmembrane, which can be displaced by pressure fields and a detectionscheme to detect membrane displacements at the pressure levels in theorder of 105 Pa. The pressure equivalent of the thermal mechanical noiseof the membrane structure should be small since this number determinesthe minimum detectable pressure level. It is preferable that a broadbandmicrophone have a membrane response free of resonances in the bandwidthof interest and a broadband detection scheme. The micro-machinedmicrophone membrane and the integrated optical detection scheme satisfythese preferred attributes. The device can also transmit sound and canbe self-calibrating by the application of bias voltages between theelectrodes 180 and grating 156. In the following description, theelements and operation of the microphone are discussed in detail.

The light source 140, in this embodiment, may be a laser, that emits anelectromagnetic wave at a known wavelength, λ. An emitted incident lightbeam 101 would be illuminated onto the reflective diffraction grating156. In this embodiment, a Helium-Neon (HeNe) laser (λ=632 nm) may beutilized. In other embodiments, the light source 140 may be a laseremitting another known wavelength. The exact wavelength of the incidentlight beam 101 may vary as long as the dimensions of the components ofthe device 100 are calculated in terms of the incident light beam 101wavelength. To that, light sources emitting more than one knownwavelength can be utilized as well, although, preferably, a light sourceemitting one known wavelength would be utilized. In practice, any kindof temporarily coherent light source with a coherence length equal to orgreater than two times the distance between the membrane 150 and thediffraction grating 156 may be utilized. The light source 140 may alsobe a Vertical Cavity Surface Emitting Laser (VCSEL) mounted on a waferor printed circuit board.

In other embodiments, the incident light beam 101 may be carried via anoptical fiber, in which case the light source 140 may be locatedremotely. As depicted in FIG. 4, the light source is positioned normalto the plane of the transparent substrate 160. Utilizing an opticalfiber adds flexibility in placing the light source 140.

In yet other embodiments, the incident light beam 101 may be guidedtowards the diffraction grating 156 via a wave guide and/or a set ofproperly placed mirrors. For instance, the light source 140 may beplaced relatively parallel to the lengthwise direction of thetransparent substrate 152. In this case, a mirror and/or a wave guidecan change the direction of the incident light beam 101 so that it isilluminated at a direction normal to the diffraction grating 156. Tothat, although it appears that the best results occur when the incidentlight beam 101 is illuminated at a direction normal to the diffractiongrating 156, it need not be necessary. Other wave shaping instrumentsmay be utilized, such as microlens, to collimate the incident light beam101. The reflected light beams 104 may also be shaped by a system ofwave shaping instruments.

The transparent substrate 160 is typically a planar surface, althoughnot necessarily. For example, the substrate 160 may be cut so as to havea rounded surface for the forming the diffraction grating 156. This mayaid in focusing the incident light beam 101. A variety of materials canbe utilized for the substrate 160. Non-limiting examples are quartz,silicon, sapphire, glass, and combinations thereof. In otherembodiments, the substrate 160 may be non-transparent, but a bulk-etchedcavity may be incorporated into the substrate 160 to allow illumination.In general, the transmission coefficient, τ, of the transparentsubstrate 160 for a given wavelength of incident light beam 101 may belarger than 0.9. The dimensions of the transparent substrate 160 canvary according to the overall structure of the device 100, but ingeneral, the lateral thickness of the substrate 160 may be in the rangeof 0.1 mm-2 mm, and likewise having a working distance of 0.1 mm-2 mmwith an F-number from 1 to 5. The lateral length can vary with thestructure of the device 100. In other embodiments, the transparentsubstrate 160 may be configured, upon manufacture, to assist in focusingthe diffracted and/or collimated incident light beams.

In an alternative embodiment, the displacement sensitivity can beimproved by fabricating a semi-transparent mirror layer on the topsurface of the transparent substrate 160.

As mentioned, the diffraction grating 156 may include several equallyspaced fingers. In general, the spatial separation between adjacentfingers may be on the order of the wavelength of the incident light beam101. The fingers may be constructed of a reflective and conductivematerial that has a reflection coefficient of between 0.8 to 1. Theconductivity of the diffraction grating fingers may be necessary forelectrostatic actuation of the fingers. In general, the fingers may beshaped as blocks and could also be composed of a conductive materialwith a non-dielectric reflective coating. In other embodiments, thefingers may be composed of a dielectric material and be coated with aconductive reflective material. The dimensions of the fingers can varygreatly with the wavelength of the incident light beam 101. In thisembodiment, however, the dimensions of the fingers may be on the orderof the wavelength of the incident light beam 101, or about 0.5 μm to 10μm. Several fingers (on the order of 10λ in lateral length) may make upthe diffraction grating 156. In this embodiment, the diffraction grating156 is formed above the top planar surface of the transparent substrate160.

The diffraction grating fingers need not be equally spaced. Thediffraction grating 156 may be configured to focus the incident lightbeam 101 to a given focal point. This may be accomplished by varying thespacing between the fingers in such a way so as to focus the light.

The electrodes 180 are placed in relation to the diffraction gratingfingers. In general, the electrodes 180 are a conductive material thatis deposited onto the substrate 160. Similar to the diffraction gratingfingers, the electrodes 180 may be a dielectric material covered with aconductive coating. The electrodes 180 may also be made of a transparentmaterial coated or doped with a conductive material.

The device 100 also includes a photo-detector 120. In this embodiment,the photo-detector 120 may be placed parallel and underneath thesubstrate 160. As the figure depicts, the photo-detector 120 may bepositioned to receive a higher diffraction order of the reflected light,such as the 1^(st) or 3^(rd) order. The observation length, x, can vary,but should be properly positioned so that a higher diffraction order maybe observed. For example, the observation length x, may vary with thewavelength of the incident light beam 101. The photo-detector 120 may beplaced at an optimal longitudinal distance, e.g. 1000 μm, but this mayvary with wavelength.

In other embodiments, the photo-detector 120 may be remotely located andthe diffracted light may be received via an appropriately placed opticalfiber. In yet other embodiments, a wave guide and/or mirrors may changethe direction of the diffracted and reflected beams. In this embodiment,as mentioned, the photo-detector 120 is placed parallel to the substrate160. This allows for a relatively small space, on the order of 100μm-1000 μm.

Several photo-detectors 120 are known in the art. In general, anyphoto-detector 120 that can be configured for micromachining and cansustain the desired bandwidth can be utilized. One specific example of aphoto-detector 120 that can be used is a silicon P-N junctionphotodiode. Another type that could be utilized is a P-I-N typephotodiode. The utilized photo-detector 120 may depend on the processingspeed and responsivity (photocurrent per Watt of incident light)requirements. For example, at wavelengths where the absorption ofsilicon is small, deeper junction depths may be used to increaseresponsivity.

Similarly, the geometry of the photo-detector 120 may be adjusted tominimize its capacitance and transit time to increase the detectionbandwidth. Some signal conditioning circuitry, such as a transimpedanceamplifier, may also be implemented on the same semiconductor substrateas the photo-detector 120 to minimize noise and decrease parasiticcapacitance. These photo-detectors 120 with integrated electronics canbe configured to operate with bandwidths from DC to GHz range forsensing optical communication applications.

A processor 130 may be included within the device 100, but more thanlikely will be communicatively coupled to the photo-detector 120 and bean external component. The processor 130 may be any type of electricalcomponent that can process the signals received by the photo-detector120. Likewise, hardware, software, and/or firmware may be utilized toproperly make the appropriate calculations. For example, a personalcomputer may be configured to process the signals received from thephoto-detector 120 in data that is compiled and calculated to producethe change in distance d as a function of time. A relatively simpledigital signal processor (DSP) or an application specific integratedcircuit (ASIC) may be utilized to perform the calculations. Theprocessor 130 may also be capable of making several other calculationsand/or perform other functions, such as calibration, laser intensitynormalization, digital filtering, and signal conditioning.

Coupled to the diffraction grating 156 and electrodes 180 is acontroller 170 for controllably adjusting, in this embodiment, theposition of the diffraction grating. This is accomplished byelectrostatic actuation which is discussed below. The controller 170 maybe communicatively coupled to the processor 130. In this manner,force-feedback approaches to measuring the distance d can beaccomplished.

The structure of the devices of the present disclosure include a phasesensitive optical diffraction grating, in which the diffraction patternis determined by the membrane 150 displacement relative to thediffraction grating 156 (which position can vary, to be discussedbelow). The incident light will be reflected back to the zeroth orderwhen the gap thickness is an integer multiple of λ/2, and to the odddiffraction orders when the gap thickness is an odd multiple of λ/4.Notably, as illustrated in FIG. 3, the change in intensity of bothorders per gap thickness is maximized in a periodic fashion in thesemultiples. Operating the device 100 at the point of maximum change inintensity (the slope of the cos and sin curves of FIG. 3) is beneficialbecause it maximizes the sensitivity of the device 100.

As illustrated in FIG. 4, the photo-detector 120 is positioned toreceive the first order, which has maximum change in intensity at oddmultiples of λ/4. Notably, when a single photo-detector is used todetect the intensity of a diffraction order the output signal willinclude the effect of laser intensity noise degrading the performance ofthe device 100. This noise can be eliminated by normalizing the outputsignal with the laser output power. By way of using arrays of thesedevices 100 to form a microphone, the noise level will be furtherreduced by spatial averaging.

In order to operate the device 100 at the maximum sensitivity and toadjust for the ambient pressure changes, there can be a DC bias voltageapplied between electrodes 180 and grating 156. Adding an AC signal tothe bias, the device 100 can be used as a regular cMUT, which cantransmit sound as well as receive acoustic signals. The DC and ACvoltages electrostatically actuate the grating fingers causing them tobend (when voltage is applied) toward the electrodes 180, thus causing achange in the distance from grating 156 to reflective membrane 150. Thischange in distance causes a change in the interference pattern produced,which can be advantageous if the electrostatic actuation is performed ina controlled manner. The controller 170 can be configured to apply theelectrical signals to perform the electrostatic actuation. Thecontroller 170 may be coupled to the processor 130 to form a feedbackloop. In this manner, the position of the grating 156 can be varied suchthat the device 100 is calibrated to measure at optimal sensitivity.Furthermore, by utilizing a modulation signal which can be applied tothe electrodes 180 and grating 156, and utilizing commonly knownfrequency lock-in methods noise can be filtered out, thus increasing theSNR of the device 100.

FIG. 5 is a diagram illustrating another embodiment of amicro-displacement measurement device 200 in accordance with embodimentsof the present disclosure. The device includes a light source 240 and aphoto-diode 220 on the surface of an opaque, rigid substrate 260. Anoptical diffraction grating 215 exists above the light source 240 and ischaracterized by alternating regions of reflective and transparentpassages 217. A backside cavity 280 formed between the grating 215 andthe substrate 260 may be sealed at some desired pressure (including lowpressures) with any gas or gas mixture, or can be open to ambientthrough an opening with desired flow resistance. Finally a reflectivemembrane 250 exists above the diffraction grating 215 that reflectslight back towards the substrate 260. The diffraction grating 215 andthe reflective membrane 250 together form a phase-sensitive diffractiongrating.

The diffraction grating 215 is formed on a silicon substrate by firstdepositing an oxide layer followed by the deposition and patterning of aconductive and reflective thin film such as polysilicon in the form of agrating. A silicon nitride layer deposition is used to encapsulate thegrating fingers and then patterned to expose the underlying oxidebetween the grating fingers. The grating is released by first etchingthe bulk silicon using a deep reactive ion etching which stops at theoxide layer. This is followed by wet etching of the oxide between thegrating fingers hence providing free space optical access to thereflective top membrane.

The reflective membrane 250 is suspended in a bridge-like structure andmay be composed of a non-conductive material, such as a stretchedpolymer membrane, polysilicon, silicon-nitride, or silicon-carbide, andthen making the material conductive in a desired region either throughdepositing and patterning a conductive material such as aluminum,silver, or any metal. Alternatively, the non-conductive material may bedoped with a conductive material. The reflective membrane 250 can alsobe coated with a reflective material, which makes aluminum or silver agood choice. In some applications such as chemical and biologicalsensors, the reflective membrane 250 can be made of a single material ora multi-layered material that change its optical properties, such asreflectivity in response to a chemical or biological agent for example apolymer film dissolving in a solvent. Similarly, the reflective membrane250 can be a micromachined cantilever made of single or layered materialthat deforms, or moves, due to thermal, chemical, or other physicalstimulus. For example, the device 200 can be configured to be aninfrared (IR) sensor by having a bimorph structure as the reflectivemembrane 250 that includes an IR absorbing outer layer and a reflectivelayer facing the light source 240. In some other applications, such as amicrophone or a pressure sensor, the reflective membrane 250 can be inthe shape of a diaphragm.

A gap 285 is formed between the rigid structure 210 and the reflectivemembrane 250. The gap 285 should be large enough to have at least oneoptimal detection sensitivity point, i.e., highest slope of the curves32, 34 and smaller than half the coherence length of the light source.The backside cavity 280 is used to adjust the effect of air on themechanical response of the membrane 250. If the cavity 280 is verysmall, the spring-like stiffness of the air will be high as compared tothe membrane stiffness and hence dominate the overall mechanicalresponse. Utilizing a large cavity, e.g. 500×500×500 μm³, the airstiffness can be reduced to negligible levels. Another function of thebackside cavity 280 is to provide optical access and longitudinaldistance for the separation of diffraction orders for detection byphoto-detectors 220.

A controller 270 may be coupled to the reflective membrane 250 and therigid structure 210 so as to apply a voltage bias to electrostaticallyactuate the reflective membrane 250. When a controlled voltage bias isapplied, various aspects of the reflective membrane 250 can be altered.First, the position, with respect to the diffraction grating 215 can bechanged. Secondly the rigidity of the reflective membrane 250 may bealtered. In many applications, moving or controlling the position of thereflective membrane 250 will be desired for self-calibration,sensitivity optimization, and signal modulation purposes. For example,if the reflective membrane 250 is a diaphragm as in the case of amicrophone or a capacitive micromachined transducer, vibrating thediaphragm to produce sound in a surrounding fluid may be desired fortransmission and self-calibration. Also, while measuring thedisplacement of the diaphragm, controlling the nominal gap height to aposition that will result in maximum possible sensitivity for themeasurement may be desired.

The substrate 260 may be a printed circuit board, a silicon wafer, orany other solid material. Furthermore, the light source 240 andphoto-diode 220 may be constructed or sourced externally and attached tothe surface or fabricated directly into the material using integratedcircuit or micromachining fabrication techniques.

The light source 240 can be an optical fiber or end of a microfabricatedwaveguide with an appropriate reflector to direct the light to thedesired location in the device 200. The light source 140 of theembodiment illustrated in FIG. 4 is similar to the light source 240here. Likewise, the photo-diode 220 serves as a specific example of aphoto-detector, as discussed in relation to FIG. 4.

The operation of the device 200 is fairly similar to that of device 100.An incident light beam 201 is illuminated upon the diffraction grating215, where in turn a first portion of the light is reflected backtowards the substrate 260. A second portion of the light is diffractedabout the diffraction grating 215 and reflected off of the reflectivemembrane 250 back toward the substrate 260. An interference pattern ofthe two reflective portions is formed. A photo-diode 220 is positionedto receive, in this embodiment, the first order of the interferencepattern. Various functions can alter the reflective membrane 250, thuschanging the interference pattern. First, the distance of the reflectivemembrane 250 relative to the diffraction grating 215 (the gap 285height) can change due to a number of reasons. One reason is due to anumber of external excitations, such as acoustic pressure causing themembrane 250 to vibrate at a particular frequency. Another externalexcitation may be because of a chemical reaction occurring on themembrane 250, such as change in the residual stress in a compositemembrane due to dissolution of one of the layers. A third externalexcitation may be an infrared light source illuminated upon the membrane250, causing the membrane 250 to change position. There are a greatnumber of possibilities that can alter the position of the reflectivemembrane 250, all of which are too many to list here. The controller 270can also be factored into the movement of the reflective membrane 250either in a modulated approach or a static approach.

Second, the reflectivity of the membrane 250 may be altered, such thatthe power level of the second portion of the reflected light is altered.This change in the power level can be detected by the photo-diode 220.The reflectivity of the membrane 250 may be altered by a chemical orbiological reaction. Examples of these reactions can be those causing acolor change or change in membrane thickness. It is well known that thereflectivity of thin films strongly depends on thickness. Thereforecorrosion, etching, or deposition of different materials on the membrane250 due to chemical or biological processes will alter the reflectedlight.

Third, the rigidity of the membrane 250, and thus the magnitude in whichthe membrane 250 will vibrate can be altered in time and measured by thephoto-diode 220. The rigidity of the membrane 250 may be a factor of anexternal force, but more than likely will be caused by an electrostaticcharge applied by the controller 270. Varying the rigidity of themembrane 250 provides for another possibility. That is to vary therigidity of the membrane 250 as a function of the acoustic pressureapplied to the membrane 250. The attempt would be to keep the membrane250 as still as possible. With this approach, one can measure thevarying voltage needed to accomplish this, and thus be using aforce-feedback approach to measure the applied acoustic pressure.Force-feedback is a well known method in the art that can typicallyproduce more accurate and sensitive results. The device 200 can be madeat such a small scale, that it is very practical to string togetherthese devices in an array or matrix structure. Being able to buildmicrophone arrays on a single substrate over a small surface area allowsfabrication of devices with closely matched responses. By measuring theacoustic pressure using several closely spaced, matched microphone arrayelements the pressure gradients may be measured accurately to implementacoustic intensity probes. Furthermore, some signal processing methodssuch as gradient flow algorithms results in significant noise reductionusing a two-dimensional array of microphone arrays with a periodicitymuch smaller than the acoustic wavelength. The embodiments of thepresent disclosure enable implementation of such closely spacedmicrophones without loss of signal fidelity.

FIG. 6 illustrates some variations to the embodiment illustrated in FIG.5. First, an air release channel 382 could be implemented in either ofthe devices 200 or 300. The air release channel 382 serves to equalizethe pressure inside the cavity 380 with the ambient pressure preventingthe collapse of the microphone membrane 350 in response to changingambient pressure which can be due to wind or other non-acoustic changesin atmospheric pressure.

Secondly, a second diffraction grating 355 can me deposited on atransparent membrane 350 rather than a membrane with a mirror-likeuniform reflective surface as in the previous embodiments. The grating355 on the membrane 350 has the same periodicity as the first, thereference, diffraction grating 315, but is offset and has diffractiongrating fingers whose width is smaller than the gap between the fingersof the reference diffraction grating 315. This structure allows some ofthe incident light to transmit through the whole device 300 and alsointroduces new diffraction orders in the reflected field. This producesa new kind of phase grating.

To understand the operation one can consider the phase of the lightreflected from the reference grating (φ₁) and the grating on themembrane (φ₂). When the difference between φ₁ and φ₂ is 2 kπ, k=0, 2, 4,. . . , the apparent period of the grating is Λ_(g) (apparentreflectivity of 1, 0, 1, 0 regions assuming perfect transmission throughthe transparent membrane 350) and the even diffraction orders arereflected with angles

${{\sin\left( \theta_{n} \right)} = {n\frac{\lambda}{\Lambda_{g}}}},{n = 0},{\pm 2},{{\pm 4}\mspace{14mu}...}$In contrast, when the difference between φ₁ and φ₂ is mπ, m=1, 3, 5, . .. , the apparent period of the grating is 2Λ_(g) (apparent reflectivityof 1, 0, −1, 0, 1 regions assuming perfect transmission through thetransparent membrane 350) and the odd diffraction orders are reflectedwith angles

${{\sin\left( \theta_{n} \right)} = {n\frac{\lambda}{2\Lambda_{g}}}},{n = 1},{\pm 3},{{\pm 5}\mspace{14mu}...}$

Here it is assumed that the width of the reflective grating fingers onthe reference grating 315 and the grating 355 on the membrane 350 arethe same. This does not have to be the case if the interfering beams gothrough different paths and experience losses due to reflection atvarious interfaces and also incidence angle variations. The diffractiongrating geometry can then be adjusted to equalize the reflected orderintensities with optimized interference.

Note that in this embodiment, the intensity of the odd and even numberedorders change with 180° out of phase with each other when the gapbetween the reference and sensing diffraction grating changes. The evennumbered diffraction orders are in phase with the zero order reflection.The advantage of having other off-axis even diffraction orders in phasewith the specular reflection is that it enables one to easily usedifferential techniques. This is achieved by taking the difference ofthe outputs of two detectors (i.e., photo-diodes 320 and 322) to detectodd and even orders, respectively. Hence, the common part of the laserintensity noise which is common on both orders can be eliminated.

Similar electrostatic actuation techniques can be applied by thecontroller 370. Furthermore, similar to the device 200 of FIG. 5, thedevice 300 can be fabricated at such a small scale that it is veryfeasible to arrange many devices 300 in an array or matrix structure.

FIG. 7 illustrates yet another embodiment of the present disclosure. Inthis embodiment, a reflective diffracting grating 455 is located on atransparent membrane 450. A platform 410 includes a reflective surface,and the device 400 forms a phase sensitive diffraction grating whenilluminated from the bottomside of the membrane 450 with an incidentlight beam 401. The zero and all odd orders are reflected back and haveintensities that depend on the distance between the grating 455 and theplatform 410. Note that this gap includes the thickness of the membrane450, which may be made of any transparent material. Examples includesilicon dioxide, silicon nitride, quartz, sapphire, or a stretchedpolymer membrane such as parylene.

Since the only potential requirement of the platform 410 is to bereflective, any material including semiconductor substrates or plasticscan suffice given that they are coated with a reflective layer, such asmetal. To add the electrostatic actuation feature, a region of both theplatform 410 and the membrane 450 can be made electrically conductive.For the membrane 450, this can be accomplished by using a material thatis both reflective and conductive for the diffraction grating 455.

This particular embodiment offers remote sensing capabilities, as thelight source, as well as any photo-detectors, may be located remote fromthe device 400. For example, if measuring the displacement of themembrane 450 due to a change in pressure is desired (as would be thecase for a pressure sensor or a microphone), the platform 410 can beattached to a surface and the light source and detectors can bestationed in a remote location not necessarily close to the membrane450.

In addition to remote measurements, this device 400 can be remotelyactuated to modulate the output signal. For example, an acoustic signalat a desired frequency can be directed to the membrane 450 and theoutput signal can be measured at the same frequency using a method suchas a lock-in amplifier. The magnitude and phase of the output signalwould give information on the location of the membrane 450 on theoptical intensity curve of FIG. 3, which in turn may depend on staticpressure, and other parameters such as temperature, etc. Similarmodulation techniques can be implemented using electromagneticradiation, where an electrostatically biased membrane 450 with fixedcharges on it can be moved by applying electromagnetic forces. In thiscase, the membrane 450 would be made with a dielectric material with lowcharge leakage.

FIG. 8 is a flow chart illustrating a method 800 for optimizing thesensitivity of a micro-displacement measurement device and moregenerally improving the overall performance of a device in accordancewith embodiments of the present disclosure.

The method 800 begins with measuring the distance d (block 805). Oncethe distance is measured by the device, the measurement data can beprocessed so that the distance can be calculated (block 840). Typically,this can be performed by a processor, which may be a computing devicesuch as a personal computer. The processor may be configured to performseveral other functions with the data as well.

Either before or after the data is calculated, a control signal may begenerated based on the measured and/or calculated data to actuatefingers of a tunable diffraction grating of the device (block 850) oractuate the reflective membrane. Once the device has been appropriatelyactuated, the measurements may be made again. This procedure maycontinue, as the feedback loop provides for constant monitoring.

As described in sufficient detail in prior figures, several embodimentsof devices can properly measure the distance d, and the change of d intime. A simplified description of the general method of measuring thedistance may begin with illuminating the reflective membrane with anincident light beam through the tunable diffraction grating (block 810).Once illuminated, interference patterns can develop through constructiveand destructive interference of reflected light off of the tunablediffraction grating and reflections off of the membrane that has beendiffracted through the diffraction grating. A photo-detector may thenreceive the interference pattern (block 820). Once received properelectrical components working with the photo-detector or included withinthe photo-detector may then measure the intensity of light of theinterference patterns (block 830). Interpretation of the interferencepatterns may come in subsequent calculations.

From either the measured intensity of the interference patterns or fromprocessed calculations, a voltage potential may be generated thatcarries proper actuation information. The voltage potential may carry aDC portion that can deform chosen fingers of the tunable diffractiongrating to certain predetermined optimum positions. Alternatively, thereflective membrane may be actuated and moved to a position thatproduces optimum sensitivity (block 854). Upon actuation, an AC portionof the voltage potential may also be supplied that can act as acalibration signal during sensitivity optimization. For example, the DCbias can be changed until the diffracted light intensity variation atthe AC signal frequency is maximized for a certain AC displacement. TheAC signal can also act as a modulation signal to vary the distance d ata known frequency (block 852). As a result, upon, calculation, knownfrequency lock-in detection techniques can be used to lock in at themodulating frequency. Any vibrations from noise can thus be overcome,because, in essence they are modulated out. Improving the sensitivity ofthe device and eliminating noise in the system can help optimized theperformance of the device.

According to some embodiments of the disclosed devices, the sensorstructure can be modified to enhance desired performancecharacteristics. For example, the mechanical sensitivity for thedisclosed sensor structures can be defined as the maximum displacementof the membrane for a given pressure. The membranes described thus farhave generally been described as being uniform in thickness, and themembranes deflect into a concave shape when a net pressure is applied onthe top side of the membrane. Thus, such maximum displacement can bemeasured, for example, from the center of the concave membranedisplacement. Since the membrane displacement is detected optically forsuch devices, it can be desirable to have as flat of a deflectionprofile as possible, while still maintaining good mechanicalsensitivity. Said another way, it can be desirable for the reflectivesurface of the membrane to be flat, even during the deflection of themembrane.

According to one embodiment, a sensitivity increase can be achievedusing an increased gap thickness to provide an air-sealed cavity havinga volume larger than devices having relatively smaller gap thicknesses.For example, with reference to FIG. 9, a sensor 900 is disclosed havingsuch an increased gap height to reduce the stiffness of trapped airwithin the cavity, thereby increasing the mechanical sensitivity of themembrane.

More specifically, FIG. 9 illustrates a device having similar operatingprinciples of the sensor structures of the previously describedmicrophones. Similar to the devices of FIGS. 1 and 2, for example,device 900 includes a target surface 902 (which may be a reflectivesurface of a mechanical structure 904, such as a beam or flexiblemembrane) placed a distance 906 away from a reference point. In thiscase, the reference point is a reflective diffraction grating 908 formedon transparent substrate 910.

When air is trapped in the membrane-substrate gap a substantiallyair-tight sealed cavity 912 is formed. However, a reduction inmechanical sensitivity may occur due to the added stiffness caused bythe compression (or vacuum) of trapped air when the membrane isdeflected. Therefore, additional enhancements in mechanical sensitivitycan be made by reducing this effect.

The stiffness due to the air is inversely proportional to the volume ofthe cavity 912. Thus, the stiffness can be reduced by increasing the gapheight appropriately (i.e. distance 906). The sensitivity of the opticaldetection method is not adversely affected by large gap heights, so longas the coherence length requirement of the light source is met. Thus,distance 906 can be a relatively large distance in comparison to the gapthicknesses described in other embodiments such that the volume of airin cavity 912 is increased to effectively increase the sensitivity ofthe device.

However, the disclosed device embodiments that are configured toelectrostatically actuate the membrane may not operate as effectivelywith a larger gap height. That is, the electrostatic actuation forceused to move the membrane is inversely proportional to the gap heightsquared. Thus, according to some embodiments, tall electrodes 914 can beplaced at the outer regions of the structure 902. Using such aconfiguration, the membrane displacement can still be detected opticallywhile the tall side electrodes are manufactured to be tall enough, orotherwise positioned, such that they are close to the membrane to enablethe electrostatic actuation of the membrane. Such electrostaticactuation can be controlled by a controller 916, for example. Someembodiments of device 900 could include a controller for providing anelectrical bias to the reflective diffraction grating 908 as describedin previous embodiments. Such embodiments having controller forproviding electrical biases to the diffraction grating 908 and/orstructure 904 can, for example, controllably adjust a feature of thereflective diffraction grating 908 and/or structure 904.

Yet another embodiment for increasing the sensitivity of the sensordevices is to create an open-gap (i.e. open-cavity) structure. Forexample, FIGS. 10, 11, and 12 depict a device 1000 having such anopen-gap structure. Specifically, FIG. 10 is a side, cut-away view ofthe sensor 1000. FIGS. 11 and 12 provide a top-down view of device 1000,where FIG. 11 depicts a rectangular-shaped embodiment and FIG. 12depicts a circular-shaped embodiment.

Device 1000 includes a mechanical structure 1002 (i.e. a membrane orbeam) having a reflective surface as described in prior embodiments.However, unlike previous embodiments that incorporate a substantiallysealed cavity, cavity 1006 of device 1000 is provided with one or moreorifices to provide a passage for air to flow between cavity 1006 andthe surrounding environment outside of cavity 1006. Accordingly, withthis structure, the air can move freely in and out of these orifices,and the decrease in mechanical sensitivity caused by air compression orvacuum is reduced.

Additionally, because the temperature of the air in cavity 1006 canaffect the mechanical properties of structure 1002, it can beadvantageous to vent any heat buildup within the cavity. Accordingly,allowing air to freely flow into and out of the cavity through the oneor more orifices can mitigate such heat buildup (e.g. from heatintroduced by a light source).

The achieve the open-gap structure, any wall that forms the cavity 1006can include one or more orifices for providing the passage of airbetween the inside and outside of cavity 1006. For example, thestructure 1002 can be perforated with holes or slits 1008, side walls1010 can be perforated with holes or slots 1012, and/or substrate 1004can be perforated with holes or slots 1014. For embodimentsincorporating a back-side cavity (i.e. sensors 200 and 300 of FIGS. 5and 6), the air-release channel 382 of sensor 300 (FIG. 6) can beincluded to additionally provide an open gap structure in the back-sidecavity.

According to some embodiments, the orifices can be located in a positionor region of the cavity wall that will not interfere with the opticalsensing of device 1000. For example slots 1008 are positioned in aportion of the structure 1002 that is outside of the portion of thestructure 1002 used for reflecting the incident light that passesthrough reflective grating 1006 and is ultimately measured by a photodetector (not shown). Likewise, some embodiments may position slots 1014in a portion of the substrate 1004 such that the incident lighttransmitted toward the reflective diffraction grating from below thesubstrate is not adversely disturbed.

It should be understood that some embodiments of device 1000 may includea controller for providing an electrical bias to the reflectivediffraction grating 1016 and/or structure 1002, as described in previousembodiments. Such embodiments can, for example, controllably adjust afeature of the reflective diffraction grating and/or structure 1002.

It should be emphasized that the above-described embodiments of thepresent disclosure, are merely possible examples of implementations,merely set forth for a clear understanding of the principles of theinvention. Many variations and modifications may be made to theabove-described embodiment(s) of the invention without departingsubstantially from the spirit and principles of the invention. All suchmodifications and variations are intended to be included herein withinthe scope of the present invention and protected by the followingclaims.

1. A device comprising: a phase-sensitive reflective diffraction gratingfor reflecting a first portion of an incident light and transmitting asecond portion of the incident light such that the second portion of theincident light is diffracted; a mechanical structure having a firstregion and a second region, the mechanical structure positioned adistance d above the diffraction grating, the second portion of theincident light is reflected off of the first region of the structuresuch that an interference pattern is formed by the reflected firstportion and the reflected second portion of the incident light; and anelectrode extending toward, but spaced a distance away from, the secondregion of the mechanical structure.
 2. The device of claim 1, furthercomprising: a controller coupled to the electrode and to the mechanicalstructure and configured to provide a voltage bias to at least one ofthe electrode and the mechanical structure.
 3. The device of claim 2,wherein the controller is configured to electrostatically actuate themechanical structure by providing the voltage bias.
 4. The device ofclaim 1, wherein the mechanical structure forms a wall of asubstantially sealed cavity.
 5. The device of claim 4, wherein theelectrode is positioned inside the sealed cavity and between thereflective diffraction grating and a side wall of the substantiallysealed cavity.
 6. The device of claim 1, wherein the distance that theelectrode is spaced from the second region of the mechanical structureis selected to allow an electrostatic voltage to bias the structure. 7.The device of claim 1, further comprising: a controller coupled to saidreflective diffraction grating for controllably adjusting at least afirst feature of said reflective diffraction grating.
 8. The device ofclaim 7, wherein said controller controllably adjusts the distance dbetween the diffraction grating and the mechanical structure.
 9. Amethod comprising: illuminating a reflective diffraction grating with anincident light, the diffraction grating being positioned a distance dfrom a mechanical structure having a first region and a second region, afirst portion of the incident light being reflected and a second portionof the incident light transmitted through the diffraction grating suchthat the second portion of the incident light is diffracted; reflectingthe second portion of the incident light off of the first region of themechanical structure such that an interference pattern is formed by thereflected first portion and the reflected second portion of the incidentlight; and applying a voltage bias to an electrode extending toward, butspaced a distance away from, the second region of the mechanicalstructure.
 10. The method of claim 9, further comprising: actuating themechanical structure by applying the voltage bias to the electrode. 11.The method of claim 9, further comprising: controllably adjusting atleast a first feature of said reflective diffraction grating by applyingthe voltage bias to the electrode.
 12. The method of claim 11, furthercomprising: adjusting the distance d between the diffraction grating andthe mechanical structure by applying the voltage bias to the electrode.13. A device comprising: means for illuminating a reflective diffractiongrating with an incident light, the diffraction grating being positioneda distance d from a mechanical structure having a first region and asecond region, a first portion of the incident light being reflected anda second portion of the incident light transmitted through thediffraction grating such that the second portion of the incident lightis diffracted; means for reflecting the second portion of the incidentlight off of the first region of the mechanical structure such that aninterference pattern is formed by the reflected first portion and thereflected second portion of the incident light; and means for applying avoltage bias to an electrode extending toward, but spaced a distanceaway from, the second region of the mechanical structure.
 14. The deviceof claim 13, further comprising: means for actuating the mechanicalstructure by applying the voltage bias to the electrode.
 15. The deviceof claim 13, further comprising: means for controllably adjusting atleast a first feature of said reflective diffraction grating by applyingthe voltage bias to the electrode.
 16. The device of claim 15, furthercomprising: means for adjusting the distance d between the diffractiongrating and the mechanical structure by applying the voltage bias to theelectrode.
 17. The device of claim 13, wherein the mechanical structureforms a wall of a substantially sealed cavity.
 18. The device of claim17, wherein the electrode is positioned inside the sealed cavity andbetween the reflective diffraction grating and a side wall of thesubstantially sealed cavity.
 19. The device of claim 13, wherein thedistance that the electrode is spaced from the second region of themechanical structure is selected to allow an electrostatic voltage tobias the structure.